Molecular
biologists by the 1970s had deciphered the genetic code and could
spell out the sequence of amino acids in proteins. But inability
to easily read off the precise nucleotide sequences of DNA forestalled
further advances in molecular genetics and all prospects of genetic
engineering. Walter Gilbert (with graduate student Allan M. Maxam)
and Frederick Sanger, in 1977, working separately in the United
States and England, developed new techniques for rapid DNA sequencing.

Sanger and Gilbert each took advantage of recently discovered enzymes
and both methods benefited from improvements in gel electrophoresis,
a method used for imaging the order of nucleotides.

The Gilbert-Maxam method involved multiplying,
dividing, and carefully fragmenting DNA. A stretch of DNA would
be multiplied a millionfold in bacteria. Each strand was radioactively
labeled at one end. Nested into four groups, chemical reagents were
applied to selectively cleave the DNA strand along its basesadenine
(A), guanine (G), cytosine (C) and thymine (T). Carefully dosed,
the reagents would break the DNA into a large number of smaller
fragments of varying length. In gel electrophoresis, as a function
of DNA's negative charge, the strands would separate according to
lengthrevealing, via the terminal points of breakage, the
position of each base.

The Sanger method revealed the precise nucleotide
sequence of DNA by using "chain-terminating" or "poison" molecules
that revealed the positions of the bases. Single-stranded DNA was
employed. Complementary copies were synthesized with the help of
DNA polymerase. The resulting sample of DNA was divided into four
parts. To each part was added one of the four DNA bases, together
with a small percentage of the slightly altered chemical analogues.
These "dideoxy" versions of the bases, when incorporated into the
growing chain, terminate it. This process generated various lengths
of the DNA chain that, as in the Gilbert-Maxam method, revealed
the sequence of bases through gel electrophoresis.

The methods devised by Sanger and Gilbert made it possible to read
the nucleotide sequence for entire genes, which run from 1,000 to
30,000 bases long. For discovering these techniques Gilbert and
Sanger received the Albert Lasker Medical Research Award in 1979,
and shared the Nobel Prize in Chemistry in 1980.